Field of the Invention
There are many types of devices which require the transmission of an acoustic wave through a material such as bulk and surface wave resonators, delay lines, filters, and pulse compressors. In most such devices it is important that the time of travel, the frequency response of the transducer, and the phase of the acoustic wave remain constant despite variations in the temperature of the material used. Since the time of travel, frequency response, and phase vary with temperature in almost all materials, it is often necessary to place the material in a temperature-controlled environment, which is bulky, expensive, and unsuitable for applications where weight, space, or energy requirements are important, such as a ship, plane, or spacecraft.
Another approach to the problem is to use a material which has a zero temperature coefficient of delay (ZTCD) over the temperature range of interest. A ZTCD material is a material over which or through which an acoustic wave will travel in a constant period of time over a limited temperature range. If the material has a ZTCD, the frequency response of the transducer and phase of the acoustic wave will also be constant over the temperature range because the three properties are equivalent. Even if the temperature range is narrow, it is easier to keep a material within a range than to keep it at a single constant temperature.
Until now the only known material having a ZTCD over a useful temperature range was quartz. There are certain directions, and only certain directions through a quartz crystal which display the property of ZTCD (e.g., the ST cut which is a + 423/4.degree. rotated Y cut and propagation along the X axis is a ZTCD direction at 20.degree.C .+-.20.degree.C with a variance of about 10ppm at the ends of the range.) Out of all the piezoelectric materials in existence only quartz was known to have this unique, in fact, until now sui generis, property. It is not difficult to understand why the property is so unique, for as the temperature of material changes it undergoes thermal expansion or contraction and the velocity of an acoustic wave passing through it alters. One or both of these changes may be non-linear, and the changes may not necessarily be in the same direction or the same magnitude. In order for the time of travel to remain constant, changes in the volume of the material and the wave velocity must exactly cancel each other as the temperature varies, which is a highly improbable coincidence.
Although until now unique, quartz nevertheless has a number of other properties which could be improved upon. Supplies of good natural quartz crystals are being depleted and synthetic quartz crystals can only be made by a very expensive and time-consuming process. A lower acoustic velocity than that of quartz (3.16 .times. 10.sup.5 cm/sec for surface waves in the ST direction) is desirable as that would enable the devices to be made smaller. Finally, the coupling coefficient K.sup.2 of quartz is low (0.17 for ST cut) which means that quartz devices cannot efficiently transmit wide band acoustic waves. (The coupling coefficient is a measure of the efficiency with which electrical energy can be converted to acoustic energy in the crystal).
In order to find other materials possessing a ZTCD direction which may have more desirable properties than quartz, a great deal of money and scientific effort has been expended, so far, completely unsuccessfully.